1. Field of the Invention
The present disclosure relates to digital communications and, more particularly, to controlling frequency offsets of subscribers in uplink communications.
2. Description of the Related Art
Orthogonal frequency division multiple access (OFDMA) has been considered as a new multiple access method in a wireless Metropolitan Area Network (MAN) such as IEEE 802.16a, and as a broadband mobile Internet access network such as flash-type OFDMA. The OFDMA method uses a larger number of subcarriers than a conventional Orthogonal Frequency Division Multiplexing (OFDM) system such as IEEE 802.11a, for example, and allocates a subchannel constructed of a set of a part of the subcarriers to each subscriber. The OFDMA method enables two-dimensional resource allocation in time and frequency domains. In addition, the OFDMA can reduce overhead due to a long guard space required for high-speed transmission because the OFDMA uses a large number of subcarriers. Moreover, the OF DMA can concentrate power on a part of the subcarriers so that efficient control and service area extension can be achieved.
FIG. 1 illustrates a multiple access communications environment of an OFDMA system, indicated generally by the reference numeral 100. Referring to FIG. 1, a single cell includes a single base station BS and a plurality of subscriber stations SS-1, SS-2 and SS-M corresponding to multiple access users. The subscriber stations transmit/receive communications data via the base station. The direction from the base station to the subscriber stations is called downlink and the direction from the subscriber stations to the base station is called uplink.
FIG. 2 illustrates a frame structure in the OFDMA environment, indicated generally by the reference numeral 200. The base station includes data items to be sent to the multiple subscriber stations in a single downlink frame, and transmits the downlink frame to the subscriber stations. Each subscriber station that has received the downlink frame reproduces only data allocated thereto and processes the reproduced data. When the downlink frame is finished, each subscriber station transmits an uplink frame to the base station. Each subscriber station transmits uplink data through a subchannel or subcarrier allocated thereto. The uplink frames transmitted from the subscriber stations are requested to simultaneously arrive at the base station.
Referring to FIG. 2, each of the downlink frames and uplink frames starts with a preamble. The preamble is used for a receiver to detect and reproduce a received signal. A header of the downlink frame includes frame construction information, and a downlink map DL-MAP stores information about which subscriber station receives a data part of the downlink frame. An uplink map UL-MAP of the downlink frame stores information about which subscriber station is allocated how many subchannels through the uplink frame following the downlink frame.
FIG. 3 illustrates a downlink communications environment, indicated generally by the reference numeral 300. Referring to FIG. 3, a baseband processor 310 of the base station modulates the downlink frame of FIG. 2 and loads the modulated downlink frame on a carrier frequency fc to transmit it to respective baseband processors 320, 330 and 340 of the subscriber stations. Respective receivers of the baseband processors 320, 330 and 340 of the subscriber stations receive and demodulate the modulated downlink frame. The baseband processors 320, 330 and 340 receive the downlink frame having phases shifted by frequency offsets Δfo,1, Δfo,2 and Δfo,M of their receivers, respectively. Accordingly, the respective receivers of the baseband processors 320, 330 and 340 estimate their frequency offsets through frequency offset estimators 321, 331 and 341 and subtract the estimated frequency offsets from the received downlink frame. Then, a frequency-offset-compensated downlink frame is respectively processed by modems 322, 332 and 342 of the subscriber stations.
FIG. 4 illustrates an uplink communications environment, indicated generally by the reference numeral 400. Referring to FIG. 4, respective transmitters of the baseband processors 320, 330 and 340 of the subscriber stations load data items, respectively processed by the baseband processors 320, 330 and 340, on the carrier frequency fc to construct an uplink frame and transmit the uplink frame to the baseband processor 310 of the base station. Here, the respective transmitters of the baseband processors 320, 330 and 340 of the subscriber stations transmit data items having phases shifted by frequency offsets Δfo,1, Δfo,2 and Δfo,M of the transmitters, respectively. The baseband processor 310 of the base station receives the uplink frame, obtains an average frequency offset Δfo,BS of the frequency offsets Δfo,1, Δf0,2 and Δfo,M of the transmitters using a frequency offset estimator 311, and subtracts the average frequency offset Δfo,BS from the received uplink frame such that the baseband processor 310 processes the average-frequency-offset-compensated uplink frame.
From constellations of signals transmitted from the baseband processors 320, 330 and 340 to the base station 310, it can be known that the average frequency offset compensation of the base station deteriorates communications performance.
FIG. 5 shows the results of reproduction of an uplink frame, indicated generally by the reference numeral 500, carried out by the base station, when the uplink frame is transmitted from four subscriber stations to the base station. Assume that the frequency offsets of first, second and third subscriber stations SS1, SS2 and SS3 are set to 0, the frequency offset of a fourth subscriber station SS4 is set to 0.05 times a subcarrier interval, and the uplink frame is transmitted as a binary phase shift keying (BPSK) signal. In addition, a single frame has a 15 OFDM symbol length.
Referring to FIG. 5, the base station estimated frequency offsets using preambles of the uplink frame. However, the base station estimated intermediate values of the frequency offsets so that signals transmitted from all subscriber stations were affected. Particularly, a value received from the fourth subscriber station SS4 has a large frequency offset so that the phase of the received value is rotated for each symbol, as shown in FIG. 5(d). That is, the frequency offset of each subscriber station is not correctly compensated to generate inter-carrier-interference. This deteriorates communications performance and decreases a maximum communications transmission rate.
Therefore, what is desired is an OFDMA system and a frequency offset compensating method capable of correctly compensating for the frequency offset of each subscriber station in uplink communications.